1. Field of the Invention
The present invention relates generally to a method and system of monitoring a biological signal by radio. More particularly, the present invention relates to a method and system of monitoring a respiratory signal by radio, in which a respiratory frequency and a lung volume are precisely measured using an elastic device fastened around the abdomen without the trouble of measuring the biological signal, the respiratory signal, which is most frequently measured for inpatients, through an oral cavity.
2. Description of the Prior Art
In general, respiration is a physiological function that supplies fresh air (oxygen) into the body and then releases a byproduct, carbon dioxide, of the metabolism out of the body, and thus is essential for life. The respiration, blood pressure, pulse, and body temperature are important biological signals showing a vital sign, and thus are the highest measurement frequency of biological signals that must be measured three or four times for all the inpatients of the hospital from day to day. For this reason, whether or not the respiration occurs or measuring and monitoring an amount of respiration is very important medically.
Up to now, respiratory airflow transducer, respiratory inductive plethysmography, contactless respiration measurement, and breathing air temperature measurement have been used or studied for sensing, measuring, and monitoring of a breathing signal.
As illustrated in FIG. 1a, the respiratory airflow transducer converts an amount of air, which is inhaled when a testee closes the nose to breathe through the mouth with a breathing pipe 11 held in the mouth, into an electrical variable through a flow sensor 12 connected with the breathing pipe 11, and measures an amount of respiration using the electrical variable converted by the flow sensor 12. However, the respiratory airflow transducer is troublesome because the testee must breathe with the breathing pipe 11 held in the mouth. As such, the respiratory airflow transducer is used for a clinical spirometry test that must continuously measure respiratory airflow with precision.
As illustrated in FIG. 1b, the respiratory inductive plethysmography is a technique of measuring a change of the skin without the trouble of holding the breathing pipe in the mouth of a testee, thereby estimating a lung volume. In other words, the lung volume is estimated by contraction and expansion of the lung. More specifically, the lung volume is estimated by measuring and summing up changes of the peripheries of the thorax and abdomen caused by the respiration on the basis of a principle that the respiration causes the volumes of the thorax and abdomen to be changed.
Elastic bands, in which thorax and abdomen coils 21 and 22 of conductive metal are disposed in a zigzag shape, are fastened to the thorax and abdomen of the testee, respectively. As the peripheries of the thorax and abdomen of the testee, to whom the thorax coil 21 and the abdomen coil 22 are attached breathes, are varied while the testee breathes, a distance between the adjacent crests (or roots) of each zigzag coil is varied or displaced. Thereby, the inductances 23 of the thorax and abdomen coils that are attached to the thorax and the abdomen are changed and measured electrically. At this time, although the lung volumes are equal to each other, the contributions of the thorax and the abdomen to the lung volumes are dependent on the testee. Thus, the relative contributions k1 and k2 of each testee are calculated and applied in advance.
However, the respiratory inductive plethysmography is difficult to handle, and furthermore is impossible to wash with water, because the separate elastic bands must be fastened on the clothes and because the metal coils are attached in the elastic bands. Further, because the AC signal is required to measure the change of the inductance, a signal extracting circuit, which includes circuits of generating and measuring the AC signal having constant frequency and amplitude, becomes complicated.
As illustrated in FIG. 1c, the contactless respiration measurement is a technique for detecting respiration with no contact between a device and the body, and makes use of the fact that during respiration, the skin of the thorax moves backwards and forwards to undergo displacement. More specifically, a wave generator 31 generates waves such as ultrasonic waves or electromagnetic waves, and then sends the waves to the front of the body. Thus, the waves are reflected from the body. At this time, a wave detector 32 detects properties of the reflected waves, and compares the reflected waves with the incident waves. Thereby, the displacement caused by the periodic motion of the physical skin is measured. However, the wave signals are greatly attenuated in the air, and thus become weak. As a result, a quality of measurement becomes very bad. Furthermore, if the waves are not accurately emitted to the front of the body in the direction perpendicular to the front of the body, such measurement is impossible. The wave generator and detector are difficult to produce in the technical aspect, and thus have high production cost. Due to this problem, the contactless respiration measurement is under the development, and thus has a small possibility of practical use.
As illustrated in FIG. 1d, the breathing air temperature measurement is a technique based on the facts that, because a room temperature and a body temperature are is about 25° C. and about 37° C. respectively, a difference between the room temperature and the body temperature is about 10° C., and that, because a temperature of expiratory air when a testee exhales is equal to the body temperature, it is higher than that of inspiratory air when the testee inhales.
When a sensor (e.g. thermocouple or thermistor) 41 for sensing a temperature is located near the nostrils of the testee, a period of temperature change is equal to a respiratory period, so that it can be calculated to measure a respiratory frequency. However, this technique can measure only the respiratory frequency, but not a variable related to ventilation (i.e. the volume of air breathed in and out of the lungs) such as a lung volume. According to the normal physiological function of the body, the lung volume is increased first when the metabolism of the body increases to require increasing the ventilation, and then the respiratory frequency is increased only for still greater ventilation. Taking this fact into consideration, the measurement of the respiratory frequency makes it possible to determine whether or not the respiration occurs, but it makes it impossible to measure the ventilation that is more important than the respiratory frequency from the physiological viewpoint.